Methods and apparatus for particle reduction in MEMS devices

ABSTRACT

A method for assembling a micro-electromechanical system (MEMS) device that includes a micro-machine is described. The method comprises forming the micro-machine on a die, the die having a top surface and a bottom surface, providing a plurality of die bonding pedestals on a surface of a housing, and mounting at least one of the top surface of the die and components of the micro-machine to the die bonding pedestals such that a bottom surface of the die at least partially shields components of the micro-machine from loose gettering material.

BACKGROUND OF THE INVENTION

This invention relates generally to manufacturing of MicroElectromechanical System (MEMS) devices, and more specifically to,getter devices and problems caused by gettering materials within MEMSdevices.

Micro-electromechanical systems (MEMS) include electrical and mechanicalcomponents integrated on the same substrate, for example, a siliconsubstrate. Substrates for MEMS devices are sometimes referred to asdies. The electrical components are fabricated using integrated circuitprocesses, while the mechanical components are fabricated usingmicromachining processes that are compatible with the integrated circuitprocesses. This combination makes it possible to fabricate an entiresystem that fits within a chip carrier using standard manufacturingprocesses.

One common application of MEMS devices is utilization within inertialsensor. The mechanical portion of the MEMS device provides the sensingcapability for the inertial sensor, while the electrical portion of theMEMS device processes the information received from the mechanicalportion. One example of an inertial sensor that utilizes a MEMS deviceis a gyroscope.

The MEMS production process involves the placement of the operationalportion of the MEMS device, sometimes referred to as a micro-machine,within a chip carrier or housing, which is then hermetically sealed.Getters are sometimes attached to the housing to facilitate removal ofwater vapor and hydrogen, for example.

Getters can however, release particles that can interfere with operationof the MEMs device. In one example, a MEMS gyroscope and other MEMSbased inertial devices can be exposed to high-G forces that may cause anamount of particles to be released from the getter, and come intocontact with moving components of the MEMS device.

BRIEF SUMMARY OF THE INVENTION

In one aspect, a method for assembling a micro-electromechanical system(MEMS) device that includes a micro-machine is provided. The methodcomprises forming the micro-machine on a die, the die having a topsurface and a bottom surface, providing a plurality of die bondingpedestals on a surface of a housing, and mounting at least one of thetop surface of the die and components of the micro-machine to the diebonding pedestals such that a bottom surface of the die at leastpartially shields components of the micro-machine from loose getteringmaterial.

In another aspect, a micro-electromechanical system (MEMS) device isprovided that comprises a micro-machine comprising a die and at leastone each of a proof mass, a motor drive comb, a motor pick-off comb, anda sense plate. The MEMS device also comprises a housing configured tohold the micro-machine, a cover to be attached to the housing, to form asubstantially sealed cavity, and a getter within the substantiallysealed cavity. The micro machine is attached to the housing such thatthe die shields the proof mass, the motor drive comb, the motor pick-offcomb, and the sense plate from particles that become dislodged from thegetter.

In still another aspect, a micro-electromechanical system (MEMS)gyroscope is provided that comprises a housing and a micro-machinecomprising a die, at least one sense plate, at least one proof masssuspended a distance from said at least one sense plate, at least onemotor drive comb and at least one motor pick-off comb. The gyroscopealso comprises a getter comprising gettering material and a coverattached to the housing forming a substantially sealed cavity for themicro-machine and getter. The die is mounted within the cavity such thatany gettering material that becomes dislodged from the getter is atleast partially prevented from contacting the sense plates, the proofmasses, the motor drive combs, and the motor pick-off combs.

In yet another aspectt, a method for mounting a micro-machine portion ofa micro-electromechanical system (MEMS) within a housing portion of theMEMS that also contains a getter is provided. The method comprisesforming a micro-machine on a die and orienting the micro-machine withinthe housing such that the die is between the getter and components ofthe micro-machine.

In yet still another aspect, a micro-electromechanical system (MEMS)accelerometer is provided that comprises a housing, a micro-machine, agetter, and a cover attached to the housing. The micro-machine comprisesa die, at least one sense plate, at least one proof mass suspended adistance from the at least one sense plate, at least one motor drivecomb and at least one motor pick-off comb. The getter includes agettering material, and the cover and housing are configured to form asubstantially sealed cavity for the micro-machine and getter. The die ismounted within the cavity such that gettering material that becomesdislodged from the getter is substantially blocked from contacting thesense plates, proof masses, motor drive combs, and motor pick-off combs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a known MEMS device utilizing a getter.

FIG. 2 is a side view of a MEMS device where the micro-machine ismounted in a flipped configuration.

FIG. 3 is a schematic view of a MEMS gyroscope which can be producedutilizing the micro-machine described with respect to FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram of one known embodiment of a Micro-ElectromechanicalSystem (MEMS) 100. MEMS 100 includes a housing 102 (sometimes referredto as a chip carrier) to which a cover 104 is eventually attached inorder to form a sealed cavity. Electrical leads 106 provide electricalconnections to a micro-machine 108 which includes a die 110 that isattached to housing 102. As shown in FIG. 1, electrical connections 109are provided through housing 102 to external devices (not shown). Forexample, in the case of a MEMS tuning fork gyroscope, micro-machine 108includes, proof masses 114, motor drive combs 116, and motor pick-offcombs 118. Micro-machine 108 further includes sense plates 120 whichform parallel plate capacitors with proof masses 114. In one embodiment,sense plates 120 are metal films that have been deposited and patternedonto die 110. Die 110 is attached to a bottom surface 122 of housing 102utilizing contacts 124. Contacts 124 are sometimes referred to as diebonding pedestals. In one embodiment, the attachment of die 110,contacts 124, and housing 102 is accomplished utilizing athermocompression bonding process or another known bonding process.

Upon attachment of micro-machine 108 to housing 102, cover 104 isattached to housing 102 to form a substantial hermetic seal. In oneembodiment, a cavity 126 is formed when cover 104 is attached to housing102. Cavity 126 is first evacuated to remove any gases (i.e. oxygen,hydrogen, water vapor) within cavity 126. Cavity is then backfilled witha dry gas to a controlled pressure. Typically the dry gas is an inertgas, for example, nitrogen or argon. In another embodiment, cover 104 isattached to housing 102 under vacuum conditions, and a vacuum is formedwithin cavity 126. Cavity 126 provides an environment that allowscomponents of micro-machine 108 to move freely. For example, proofmasses 114 may be movably coupled to die chip 110 and therefore mayoscillate within the vacuum of cavity 126.

However, the seal between housing 102 and cover 104 is typically notabsolute. In one embodiment, a getter 130 which includes a getteringmaterial (not shown) is attached to a getter substrate 132. Gettersubstrate 132 is then attached to cover 104. Getter 130 removes watervapor or other gases (e.g. hydrogen) within cavity 126, as is known inthe art. These gases are known to permeate the seal between housing 102and cover 104 over time and are also known to be emitted over time (intocavity 126) by the materials which make up housing 102 and cover 104.Removal of the water vapor and gases facilitates maintaining theenvironment within cavity 126. The gettering material of getter 130 istypically particle based, and as described above, some getteringmaterial may break free from getter 130.

FIG. 2 illustrates a side view of a MEMS device 200 that includes ahousing 200 onto which a cover 204 is attached to provide asubstantially sealed cavity 206. MEMS device 200 includes amicro-machine 208 that is attached to housing 202 in a flippedconfiguration. The term flipped, as used herein, refers to a mountingorientation of a micro-machine within a housing which is upside down ascompared to known mounting orientations. Micro-machine 208 includes adie 210, proof masses 214, motor drive combs 216, and motor pick-offcombs 218. Micro-machine 208 further includes sense plates 220 whichform parallel plate capacitors with proof masses 214. In one embodiment,sense plates 220 are metal films that have been deposited and patternedonto die 210. Proof masses 214, motor drive combs 216, motor pick-offcombs 218, and sense plates 220 are mounted onto die 210 utilizing knownprocesses. However, rather than mounting a bottom surface 222 of die 210directly to die bonding pedestals, as is done in known MEMS devices,micro-machine 208 is flipped over before being attached to the diebonding pedestals, and therefore other portions of micro-machine 208 areattached to the die bonding pedestals, as further described below.

As shown in FIG. 2, motor drive combs 216 and a top surface 224 of die210 is attached to die bonding pedestals 226, which are located on abottom surface 228 of housing 202, typically through a thermocompressionbonding process. In one embodiment, die bonding pedestals 226 are goldcontacts. By flipping micro-machine 208, die 210 is also flipped, andbottom surface 222 of die 210 provides protection for operational andmoveable portions (e.g. proof masses 214 and sense plates 220, andportions of motor drive combs 216 and motor pick-off combs 218) of micromachine 208. Protection is provided such that particles of getteringmaterial which become dislodged from getter 230, for example, due tovibration, are blocked from components of micro-machine 208, due to theorientation of micro-machine 208 with respect to getter 230.

Orientation of die 210 and arrangement of die bonding pedestals 226 alsoallows such pedestals to be utilized as electrical contacts forcomponents of micro-machine 208. Referring again to FIG. 2, pedestals240 are in contact with electrical nodes 242 on die 210, and pedestals244 are in electrical contact with motor drive combs 216. Pedestals 240and 244 also provide electrical contact with circuits outside of housing202, for example, through one of a plurality of electrical conductors250. One electrical conductor 250, is illustrated as providing anelectrical path from bottom surface 228 of housing 202 to an exteriorsurface 254 of housing 202. A number of such electrical connectionsutilizing electrical conductors similar to conductor 250 are furtherdescribed with respect to FIG. 3 below. Additional connections to suchconductors can be made to components of micro-machine 208 withadditional pedestals 226.

MEMS device 200 may comprise more or fewer components than described.For instance, while four electrical connections are illustrated (e.g.four pedestals 226), those skilled in the art will recognize that a MEMSdevice may comprise more than two contacts and/or extruding pins aswell. Additionally, more or fewer members (proof masses, drive combs,pick-off combs, etc.) may be present in MEMS device 200 other than thosecomponents above described. Further, components of MEMS device 200 maycomprise multiple functions. Micro-machine 208 may be any suchelectromechanical machine used in accordance with MEMS and MEMS baseddevices. In addition, alternate packages may be used as well to providea housing for MEMS device 200. The illustrations in the Figures areintended to show embodiments for mounting a micro-machine that providesprotection from dislodged gettering material rather than provide adescription of a specific MEMS device.

FIG. 3 is a schematic illustration of a MEMS gyroscope 400 whichincorporates a micro-machine oriented similarly to micro-machine 208,described with respect to FIG. 2. Such an orientation has been referredto herein as a flipped or upside down orientation. MEMS gyroscope 400includes a housing 402 (similar to housing 202 (shown in FIG. 2)) thatincludes therein a micro-machine which is a tuning fork gyroscope (TFG)404. Housing 402 is sealed with a cover (not shown). Housing 402 may bea plastic package, a small outline integrated circuit (SOIC) package, aceramic leadless chip carrier, a plastic leaded chip carrier (PLCC)package, a quad flat package (QFP), or other housings as known in theart. Housing 402 may provide a structure to co-locate elements of TFG404 and/or locate other elements within a close proximity of one anotherwithin the housing 402. TFG 404, in one embodiment, is located within asubstantially sealed cavity 406 which is formed by bonding the cover tohousing 402.

In one embodiment, TFG 404 includes proof masses 414, motor drive combs416, motor pick-off combs 418, and sense plates 420 constructed on awafer. A pre-amplifier 422 may be included within housing 402 and iselectrically connected or coupled to each proof mass 414 and sense plate420 combination, for example, through die bonding pedestals 226 (shownin FIG. 2). Pre-amplifier 422 and TFG 404 may both be formed on a commonsubstrate and, in one embodiment, are electrically connected. In otherembodiments, pre-amplifier 422 is electrically connected to proof masses414. An output of pre-amplifier 422 is sent to sense electronics 424, oralternatively, pre-amplifier 422 may be incorporated within senseelectronics 424.

In addition, an output 426 of motor pick-off combs 418 is transferred tofeedback monitors 428. Feedback monitors 428 provide output signals 430to drive electronics 432, which power motor drive combs 416.Alternatively, feedback monitors 428 may be incorporated within driveelectronics 432. MEMS gyroscope 400 may also include a system powersource and other operational electronics, which are not shown in FIG. 3for ease of illustration.

In other embodiments (not shown) one or more of pre-amplifier 422, senseelectronics 424, feedback monitors 428, and drive electronics 432 may bemounted on bottom surface 222 (shown in FIG. 2) of die 210 (shown inFIG. 2). To make electrical connections between these components andcomponents external to housing 202 (shown in FIG. 2), housing 202 can beconfigured with electrical leads 106 and electrical connections 109,similar to those shown in FIG. 1.

Motor drive combs 416 excite the proof masses 414 using electrostaticforces by applying a voltage to electrodes of proof masses 414. Motorpick-off combs 418 monitor the excitation or oscillation of proof masses414 by monitoring voltage signals on electrodes on proof masses 414.Motor pick-off combs 418 output a feedback signal to feedback monitors428. Feedback monitor 428 provides an output 430 which is input to driveelectronics 432. If proof masses 414 begin to oscillate too fast or tooslow, drive electronics 432 may adjust an oscillation frequency suchthat proof masses 414 vibrate at a resonant frequency. Excitation atsuch a frequency may enable a higher amplitude output signal to begenerated. Many or all of the above described electricalinterconnections may be accomplished utilizing die bonding pedestalswhen the micro-machine is in a flipped configuration.

While operation of gyroscope 400 is described, such operation is notlikely if particles of gettering materials, for example, as describedabove, are released within cavity 406. By orienting the micro-machine inan upside down or flipped configuration, a secondary cavity isessentially obtained which substantially reduces probabilities ofgettering particles coming into contact with components, of themicro-machine, including those components which need to be able to movefreely for proper operation.

Such a flipped micro-machine configuration is further usable in othersensor based-devices. It is contemplated to utilize the flippedmicro-machine orientation and method described herein in a variety ofMEMS devices, including, but not limited to, MEMS inertial measurementunits, gyroscopes, pressure sensors, temperature sensors, resonators,air flow sensors, and accelerometers.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A micro-electromechanical system (MEMS) device comprising: amicro-machine comprising a die and at least one each of a proof mass, amotor drive comb, a motor pick-off comb, and a sense plate; a housingfor said micro-machine; a cover configured to be attached to saidhousing, said cover and said housing forming a substantially sealedcavity; and a getter within the substantially sealed cavity, said micromachine attached to said housing such that a surface of said die isbetween micro-machine and said getter to shield said proof mass, saidmotor drive comb, said motor pick-off comb, and said sense plate fromparticles that become dislodged from said getter.
 2. A MEMS deviceaccording to claim 1 further comprising a plurality of die bondingpedestals said micro-machine attached to said die bonding pedestals. 3.A MEMS device according to claim 2 wherein said micro-machine isattached to said die bonding pedestals utilizing a thermocompressionbonding process.
 4. A MEMS device according to claim 1 comprising aplurality of electrical conductors passing through said housing, saiddie bonding pedestals in contact with said electrical conductors.
 5. AMEMS device according to claim 1 wherein said MEMS device is one of agyroscope, an accelerometer, an inertial measurement unit, a pressuresensor, a resonator, an air flow sensor, and a temperature sensor.
 6. Amicro-electromechanical system (MEMS) gyroscope comprising: a housing; amicro-machine comprising a die, at least one sense plate, at least oneproof mass suspended a distance from said at least one sense plate, atleast one motor drive comb and at least one motor pick-off comb; agetter comprising gettering material; and a cover attached to saidhousing, said cover and said housing configured to form a substantiallysealed cavity for said micro-machine and said getter, said die mountedwithin the cavity such that gettering material that becomes dislodgedfrom said getter is substantially blocked from contacting said senseplates, said proof masses, said motor drive combs, and said motorpick-off combs by a surface of said die.
 7. A MEMS gyroscope accordingto claim 6 further comprising a plurality of die bonding pedestalsattached to said housing, said micro-machine attached to said diebonding pedestals.
 8. A MEMS gyroscope according to claim 7 wherein atleast a portion of electrical connections to said die, said senseplates, said proof masses, said motor drive combs, and said motorpick-off combs are made through said plurality of die bonding pedestals.9. A MEMS gyroscope according to claim 7 wherein said housing comprisesat least one electrical conductor passing through said housing, saidelectrical conductor in contact with one of said die bonding pedestals.10. A MEMS gyroscope according to claim 6 wherein a bottom surface ofsaid die is between said getter and said proof masses, said motor drivecombs, said motor pick-off combs, and said sense plates.
 11. Amicro-electromechanical system (MEMS) accelerometer comprising: ahousing; a micro-machine comprising a die, at least one sense plate, atleast one proof mass suspended a distance from said at least one senseplate, at least one motor drive comb and at least one motor pick-offcomb; a getter comprising gettering material; and a cover attached tosaid housing, said cover and said housing configured to form asubstantially sealed cavity for said micro-machine and said getter, saiddie mounted within the cavity such that gettering material that becomesdislodged from said getter is substantially blocked from contacting saidsense plates, said proof masses, said motor drive combs, and said motorpick-off combs by a surface of said die.
 12. A MEMS accelerometeraccording to claim 11 further comprising a plurality of die bondingpedestals attaching said housing to said micro-machine.
 13. A MEMSaccelerometer according to claim 12 wherein at least a portion ofelectrical connections to said die, said sense plates, said proofmasses, said motor drive combs, and said motor pick-off combs are madethrough said plurality of die bonding pedestals.
 14. A MEMSaccelerometer according to claim 12 wherein said housing comprises atleast one electrical conductor passing through said housing, saidelectrical conductor in contact with one of said die bonding pedestals.15. A MEMS accelerometer according to claim 12 wherein a bottom surfaceof said die is between said getter and said proof masses, said motordrive combs, said motor pick-off combs, and said sense plates.
 16. AMEMS device according to claim 1 wherein a bottom surface of said die isbetween said getter and said micro-machine.